Skype on Saturday released an upgrade to its iPhone app that allows calls to be made and received under AT&T's 3G network, but there's a catch - they won't be free for very long, even for Skype-to-Skype calls or for people who have all-you-can-eat calling plans with the internet telephony company.

No specific pricing or even timeline was announced, but the release notes of version 2.0.0 (to the right) say this feature will be free only "until at least the end of August 2010, after which there will be a small monthly fee."

All of this became possible only after Apple changed its iPhone SDK in anticipation of the iPad.

Skype pronouced on Feb 3 that 3G calling would happen "soon." But the additional cost was not trumpeted and does come as a surprise, though perhaps not a terribly big one given the revenue possibilities and the additi…